1. Field of the Invention
The present invention relates to a method and apparatus for calibrating a marking position in a chip scale marker, and more particularly, to a method and apparatus for calibrating a marking position in a chip scale marker that marks characters on a wafer-chip with a laser.
2. Description of the Related Art
In general, each wafer used in a semiconductor manufacturing process is composed of several thousand to several ten thousand chips. Once chips are completed, a marking process of marking characters and/or numbers is performed on the completed chips so as to classify the chips according to their product lot numbers. At this time, the marking process is performed by a chip scale marker using a laser beam.
FIG. 1 is a schematic view of a general chip scale marker 10, illustrated with a wafer w. Referring to FIG. 1, the wafer w is placed on a wafer holder 20, and a laser 30 is positioned below the wafer holder 20. A laser beam is oscillated from a laser source of the laser 30, irradiated on chips on the wafer w via a plurality of mirrors (not shown) of a galvano scanner 32 and an f-theta lens 34, and finally marks as characters on the chips.
Above the wafer holder 20, a camera 40 is positioned for monitoring an object held by the wafer holder 20. The camera 40 is connected to an X-Y stage 50 and moves with the X-Y stage 50. Here, reference numeral xe2x80x9860xe2x80x99 denotes a table on which the X-Y stage 50 and the wafer holder 20 are placed.
To perform a marking process precisely on the chips on a wafer, wafer alignment has to be precisely accomplished. Here, wafer alignment means positioning a wafer at a marking position according to geometrical characteristics of the wafer or a recognition part. A marking process is performed by recognizing the recognition characteristics of a wafer, such as a ball array or a recognition mark, by optical methods, and irradiating a laser beam at the marking position using a suitable optical system. At this time, recognition of the exact chip position and precise laser beam irradiation are required to perform the marking process on a chip of less than 1 mm2. However, even if the aforementioned conditions are satisfied, the irradiated position of a laser beam may gradually change due to external conditions such as vibration and heat. Therefore, there is a need to periodically check this change and calibrate the laser beam irradiation position if necessary. Although the period of measuring and calibrating the marking position varies according to the type of tools used and working conditions, it is necessary to periodically and conveniently check if a laser beam is irradiated at a desired position.
FIG. 2 is a diagram explaining a conventional method of measuring marking errors. Conventionally, a laser beam is irradiated on a wafer-type plate 70 in which a plurality of holes 70a having a diameter of 0.3 mm are formed, and then the position of the laser beam, which passed through these holes 70a, is detected by a camera 40 so as to compare the position of the laser beam with a desired position thereof. Next, the irradiation path of the laser beam is calibrated based on the detected difference between the detected position and the desired position.
However, this conventional method is disadvantageous in that a laser beam passing through the holes 70a is monitored via a front glass 42 of the camera 40. The laser beam is irradiated at an inclination with respect to the hole 70a as indicated with the dotted line in FIG. 2 and, thus, refracted at the front glass 42 of the camera 40. For this reason, it is difficult to detect the exact irradiation position of the laser beam at the plate 70 that is placed at the same position as a wafer.
To solve the above-described problems, it is a first object of the present invention to provide a method of calibrating a marking position in a chip scale marker, by irradiating a laser beam on a semi-transparent screen rather than a wafer, detecting the irradiated position and calibrating the marking position.
It is a second object of the present invention to provide an apparatus for calibrating a marking position in a chip scale marker for the above-mentioned method.
To achieve an aspect of the first object, there is provided a method of calibrating a marking position in a chip scale marker which performs a marking process by irradiating a laser beam from a laser source on a wafer via a galvano scanner and an f-theta lens, including: (a) placing a screen which is equivalent in shape to the wafer on a wafer holder for holding the wafer; (b) irradiating a laser beam at a predetermined target point on the screen, and measuring the position of the laser beam by a camera being moved above the target point; (c) transmitting the measured position information to a controller; (d) repeating steps (b) and (c) at a plurality of predetermined points; (e) comparing the transmitted position information with the target point; and (f) calibrating the position of the laser beam irradiated on the wafer by adjusting mirrors of the galvano scanner in the event that a deviation between the position information and the target point falls beyond a predetermined value.
To achieve another aspect of the first object, there is provided a method of calibrating a marking position in a chip scale marker which performs a marking process by irradiating a laser beam from a laser source on a wafer via a galvano scanner and an f-theta lens, including: (a) unloading the wafer from a wafer holder; (b) positioning a camera screen at the front of a camera for measuring a beam position; (c) moving the camera and the camera screen to a predetermined target point; (d) irradiating a laser beam at the target point and measuring the position of the laser beam irradiated on the camera screen; (e) transmitting the measured position information to a controller; (f) repeating steps (c) to (e) at several predetermined points: (g) comparing the transmitted position with the target point; and (h) calibrating the position of the laser beam irradiated on the wafer by adjusting the position of mirrors of the galvano scanner in the event that a deviation between the position and the target point is beyond a predetermined value.
To achieve the second object, there is provided an apparatus for calibrating a marking point in a chip scale marker having wafer marking laser, a wafer holder for holding a wafer, and a camera moving while connected to an X-Y stage above the wafer holder and measuring an object held by the wafer holder. The apparatus includes a screen; and a controller for receiving information regarding the position of a laser beam irradiated on the screen and for calibrating the position of mirrors of a galvano scanner of the wafer marking laser in the event that a deviation between the laser beam position and a target point is beyond a predetermined value.
Preferably, the screen is equivalent in shape to the wafer, and the screen, on which a laser beam from the laser is irradiated and a beam point is marked, is placed on the wafer holder.